Industrial gas turbine engines, such as those used for electrical power generation or as industrial powerplants, are subject to stringent regulation of nitrous oxides (NOx) and other undesirable exhaust emissions. In order to minimize these emissions, industrial gas turbines are equipped with premixing fuel injectors that may be of the type known as tangential entry injectors. A typical tangential entry injector features an axially extending centerbody and a pair of arcuate scrolls that extend axially between a forward bulkhead and an aft bulkhead. The scrolls are radially spaced from the centerbody to bound an annular mixing chamber. The scrolls are also radially offset from each other to define a pair of air intake slots, each of which admits a stream of primary combustion air tangentially into the mixing chamber. Each scroll includes an array of axially distributed fuel injection passages for introducing primary fuel into the incoming airstream. The aft bulkhead of the injector includes a discharge port for introducing the primary fuel and air into the engine combustor, and the aftmost extremity of the port defines a fuel injector discharge plane.
The injector centerbody includes a base affixed to the forward bulkhead, an injection insert having a flat aft surface, and a substantially frustoconical hollow shell. The shell extends axially from the base to define both the radially inner extremity of the mixing chamber and the radially outer extremity of a secondary air supply conduit. The injection insert is axially spaced from the base and rests snugly within the aft end of the shell so that its aft, axially facing flat surface is axially aligned with both the trailing edge of the centerbody and with the injector discharge plane. Although the insert and the aft end of the shell are in mutual contact, the insert is fastened only to a secondary fuel supply tube that originates at the base and extends linearly through the conduit. Thus, the insert is supported radially by the aft end of the shell and axially by the secondary fuel supply tube. The absence of a positive connection between the shell and the insert protects the injector from damage by allowing the shell and insert to slide axially relative to each other in response to dissimilar, thermally induced dimensional changes. These dissimilar dimensional changes arise because the centerbody shell can reach temperatures as high as 900.degree. F., but the fuel supply tube is exposed to fuel at a temperature of no more than about 200.degree. F. Consequently the centerbody shell expands considerably in the axial direction but the fuel supply tube expands relatively little in the axial direction.
During engine operation, the primary air and fuel enter the mixing chamber, swirl around the centerbody and become intimately intermixed. The swirling fuel-air mixture flows axially through the mixing chamber, past the injector discharge plane and into the engine combustor where the mixture is ignited and burned. The thoroughly blended fuel-air mixture keeps the combustion flame temperature uniformly low, a prerequisite for NOx suppression, and promotes complete, clean combustion. Concurrently, a stream of secondary air enters the air supply conduit through holes in the base, and a secondary fuel stream flows through the fuel supply tube. The injection insert divides the secondary air and fuel streams into discrete, judiciously distributed jets of air and fuel, and introduces those jets into the combustor. The secondary fuel and air encourage the combustion flame to become anchored to and spatially stabilized by the exposed, aft end of the insert. As a result, the flame resists being ingested into the mixing chamber where it could cause considerable damage. The spatially stabilized flame also minimizes the likelihood of aero-thermal acoustic resonance, a phenomenon associated with spatial instability of the flame, and one that can cause considerable structural damage to the engine. Finally, because the aft face of the insert is axially aligned with the trailing edge of the centerbody, the anchored, spatially stabilized combustion flame burns entirely outside the centerbody, thereby preventing heat related damage to the interior of the centerbody.
Despite the many merits of tangential entry fuel injectors as described above, they are not without potential shortcomings. In particular, the absence of a positive connection between the insert and the shell, while desirable for preventing thermally induced damage, may not be completely satisfactory for extended, trouble free service. The relative sliding motion between the juxtaposed surfaces of the insert and the shell can erode those surfaces and compromise the snug fit between the insert and the shell. As the wear progresses, a narrow annulus develops between the insert and the shell so that the insert is free to vibrate. The vibrating insert can overstress and break the connection between the fuel supply tube and the centerbody base. In addition, a small but unregulated quantity of secondary air leaks through the annulus and may increase exhaust emissions or undermine the ability of the flame to remain anchored to the insert. In addition, if the fuel supply tube breaks anywhere along its length, the insert could be dislodged from the injector with the potential for causing considerable foreign object damage to the engine. Finally, the unequal axial thermal expansion of the shell relative to the fuel supply tube can cause the aft face of the insert to become axially recessed in the shell. The combustion flame, which is anchored to the aftmost surface of the insert, would then be partially recessed into the shell where the flame can cause heat related damage.
What is needed is a premixing fuel injector that accommodates dissimilar dimensional changes of the centerbody shell relative to the secondary fuel tube, exhibits superior durability, resists degradation of its operating characterizes and minimizes the risk of liberated parts and attendant foreign object damage.